As will be appreciated herein below, except as otherwise indicated, alloy designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2006.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
In general the purification of molten metals can be economically very profitable as it allows scrap metals which may contain different alloying or foreign elements and different amounts of the foreign elements to be processed to regain a standard purity and a higher economic value.
Such a purification method is known in the prior art, for example from international patent application WO-2005/095658-A1. In this method the molten metal is cooled to a eutectic temperature in order to simultaneously form purified metal crystals and crystals comprising at least one foreign element. Compared to the starting composition of the molten metal the purified metal crystals are depleted of foreign elements, while the foreign element(s) are concentrated in the other crystals comprising at least one foreign element. Then at least some of the crystals comprising at least one foreign element are separated from the purified metal crystals by using a solid-solid separation technique. This known purification method is beneficial in view of economic feasibility for e.g. purifying scrap such as recycled aluminium, because a relatively pure metal as purified metal crystals can be obtained in a high yield. The solid-solid separation step is exemplified by inter alia a liquid-solid pre-separation step wherein both types of crystals are separated from the molten metal as a mixture and then this mixture is added to molten salt with a specific density between that of the specific densities of the purified metal crystals and the crystals comprising at least one foreign element so that some of the crystals sink into the salt whilst the remainder float on the salt.
Other methods for solid-solid separation described in international patent application WO-2005/095658-A1 use centrifugal forces, an electromagnetic field and gas bubbles based floatation. Alternative pre-separation steps for separating both types of crystals from the total amount of molten metal are also described, such as filtration, centrifugation and a salt layer contacting step using stirring. In this patent document it has been recognised that solid-solid separation generally becomes difficult when the solid fraction is above 30%. In this process both types of crystals form simultaneously and settle together in the molten metal thereby forming a mixture of crystals. Although various techniques have been described for separating this mixture into its components, it has appeared that these techniques render the process amongst others complex and laborious to carry out.
Another purification method concerns the so-called fractional crystallisation as described in for example U.S. Pat. No. 4,273,627. In this method a hypo-eutectic molten metal comprising one or more foreign elements is cooled to achieve partial solidification. The molten metal is cooled to just above a eutectic temperature. The crystals that form in the molten metal have a purer composition than that of the molten metal that is used as a starting point. These crystals can then be separated from the remaining molten metal by means of a solid-liquid separation technique. This process however has the drawback that when the initial concentration of foreign elements is high the amount of purified metal obtained is relatively low and the amount of by-product generated is high. This means that the fractional crystallisation method may not be economically feasible for e.g. purifying scrap.
Another purification method is by means of separation of foreign elements in which a hyper-eutectic molten metal comprising one or more foreign elements is cooled to achieve partial solidification. The molten metal is cooled to just above the eutectic temperature. The foreign element(s) solidify to form crystals comprising at least one foreign element and/or pure crystals of a foreign element which can then be separated from the molten metal using a solid-liquid separation technique. A hypo-eutectic molten metal can be made hyper-eutectic by the addition of certain elements as disclosed in U.S. Pat. No. 5,741,348. This method has the disadvantage that the remaining liquid product obtained is not very pure and thus is of relatively low value.
And EP-1288319-A1 discloses a method for the purification of molten aluminium whereby the composition of the molten aluminium is alternated between a hypo-eutectic and hyper-eutectic composition by the addition of specific alloying elements. In particular the addition of Mn is disclosed to assist in the removal of Fe by the formation of eutectic particles or intermetallic particles which are separated out from the molten aluminium.
There is an ongoing need in the art for simplified and economical purification processes.